Wi-Fi Hot Spot is a site that offers network services over a wireless local area network for mobile users. A typical Wi-Fi Hot Spot has more than one access point (AP) occupying a wireless channel Wi-Fi Hot Spot is generally set up at train stations, shops, companies, department stores, or the likes. When one or more mobile wireless devices (called stations, STA) enter a coverage area of a Hot Spot, it has to associate with an AP which provides the service. There are two ways, active scanning and passive scanning, for an STA to discover an AP which will provide the service.
For active scanning, an STA searches APs by broadcasting one or more request messages, for example, the probe request message defined in the IEEE 802.11 standards. The APs receiving the broadcasted request messages will respond by returning a response message, for example, the probe response message in the IEEE 802.11 standards.
By using active scanning for AP discovery, one of the problems is that it will potentially create a broadcast storm. Assuming there are n STAs entering a Hot Spot area within a brief interval where the Hot Spot has m APs. If every STA sends at least one request message which is received by all APs and every AP returns at least one response message, there are at least (n×m) messages generated in the active scanning process. The time for transmitting the message and the time it takes to compete for the channel access cause lots of overhead.
An alternative approach is passive scanning. In passive scanning, an STA switches to each available channel and listens to beacon messages, management messages transmitted by an AP. In general, each AP sends a beacon message every 100 ms. If APs on the same wireless channel are uncoordinated, it is possible that two or more APs send beacon messages at the same time and end up colliding with each other without knowing about it. In addition, even with multiple APs, an STA may still have to wait for nearly a 100 ms in a worst case.
Therefore, a method for a plurality of APs to efficiently coordinate beacon transmission times to achieve a goal of fast link set up is required.